Yep, overpriced and a letdown. You said it.
This game only kept my interest for about a month before I stopped playing it regularly. This is probably the fastest that I've gotten bored of a Mario Kart game. 8 Deluxe and past games kept me hooked for much longer.
What were the problems?
I already talked about how much I hate what they did with the character roster in this game and that I think that this is the worst set of newcomers ever. Honestly, if they just copy-pasted Mario Kart 8 Deluxe's roster, I would've been much happier.
The number of race tracks... Maybe I've been spoiled by 8 Deluxe's massive 96 track count, but going back to 32 does seem kind of disappointing and is rather low. In 8 Deluxe you'd rarely play the same tracks twice or more in the same session because of how many there are, but in this game sometimes it feels like I keep getting the same race tracks over and over again. With less tracks, repeats are more common/likely...
Kart selection is also kind of weak imo.
As for the new gameplay mechanics, driving on walls and rail grinding are both cool. Water skiing and flying control like crap, though.
I also like the open world stuff and having the race tracks be connected to each other. I remember someone on GameFAQs described the new routes as feeling like you were on a road trip, and I agree. I honestly prefer racing with routes enabled than regular 3-lap races.